Air-cooled chiller systems are well known in the art and commonly used in cooling the air in commercial buildings, schools, hospitals, restaurants and the like. A conventional air-cooled chiller system includes one or more refrigeration units having respective closed loop refrigerant circuits in operative association with a cooling fluid circuit. Each refrigeration unit includes a compressor rack having one or a plurality of compressors arrayed in a parallel arrangement, an air-cooled condenser, an evaporator and an expansion device disposed conventionally in a closed loop refrigerant circuit. Refrigerant is evaporated as it passes through the respective evaporators in heat exchange relationship with cooling fluid, typically water or a glycol solution, circulating through the cooling fluid circuit. Each of the refrigerant circuits is typically charged with a conventional refrigerant, such as R22, R134A, R410A, R407C, ammonia or the like.
The air-cooled condensers of conventional air-cooled chiller systems include a heat exchanger tube coil through which high pressure, high temperature refrigerant vapor is conveyed in heat exchange relationship with ambient outdoor air passed through the heat exchanger tube coil over the refrigerant conveying tubes. One or more fans are provided in operative association with the condenser heat exchanger tube coil in either a forced air or an induced draft arrangement. The heat transfer coefficient of the condenser heat exchanger coil, and consequently the heat transfer performance of the condenser, is proportional to the rate of air flow through the condenser. In conventional practice, the operation of each refrigeration unit is controlled so that the sensed saturated discharge temperature matches a saturated discharge temperature set point, which set point is held constant over a wide range of operating conditions. Consequently, the chiller system is operated at less than its optimal energy efficiency rating over a wide portion of its operational envelope.
U.S. Pat. No. 6,272,868, of Grabon et al. discloses a method and apparatus for indicating condenser coil performance on air-cooled chillers. An indication of the degree of degradation in the heat transfer performance of an operating condenser is obtained in real-time by comparing an estimate of the actual total heat rejection of the operating condenser to a base total heat rejection representative of a clean condenser. The total heat rejection of the operating condenser is estimated using an algorithm that is a function of the saturation suction temperature, the saturated condensing temperature, and the outside air temperature.